As used herein and throughout, the terms in quotations below are defined as follows:                a. The term “bandpass filter” may refer to a filter that may pass signals at frequencies within a specified frequency band and may reject signals at frequencies above the specified frequency band, as well as signals at frequencies below the specified frequency band.        b. The term “diplexer” may refer to a three-port frequency-dependent device that may be used as a separator and/or a combiner of signals.        c. The term “highpass filter” may refer to a filter that may pass signals at frequencies above a specified frequency and may reject signals at frequencies below the specified frequency.        d. The term “lowpass filter” may refer to a filter that may pass signals at frequencies below a specified frequency and may reject signals at frequencies above the specified frequency        e. The term “multiplexer” may refer to a multi-port frequency-dependent device that may be used as a separator and/or a combiner of signals.        f. The term “triplexer” may refer to a four-port frequency-dependent device that may be used as a separator and/or a combiner of signals.        
A variety of communications systems today may operate in multiple frequency bands. For example, a mobile telephone may operate over multiple frequency bands, such as those bands for telephones operable in the EGSM (Extended Global System for Mobile communications) system and the DCS (Digital Cellular System) system. Communications systems that may operate in multiple frequency bands, e.g., two frequency bands, may employ a diplexer to (1) separate received signals into signals in multiple frequency bands for use by the communications system and/or (2) combine signals from multiple frequency bands for transmission by the communications system.
FIG. 1A shows a block diagram of a prior art diplexer 10. Diplexer 10 includes a lowpass filter 12 and a highpass filter 14, and may be characterized as employing a lowpass-highpass-filter architecture. A terminal of lowpass filter 12 is coupled to a terminal 16 and the other terminal of lowpass filter 12 is coupled to a terminal 18. Similarly, a terminal of highpass filter 14 is coupled to terminal 16 and the other terminal of highpass filter 14 is coupled to a terminal 20. A communications system (not shown) employing diplexer 10 may include an antenna (not shown) coupled to terminal 16 and signal processing circuitry (not shown), e.g., receiver circuitry and/or transmitter circuitry, coupled to terminals 18 and/or 20.
In operation, a communications system employing diplexer 10 may receive over its antenna signals in multiple frequency bands, e.g., signals in one frequency band including frequency f1 and signals in another frequency band including frequency f2. Diplexer 10 may receive and input these signals to lowpass filter 12 and to highpass filter 14. As represented in FIG. 1B, lowpass filter 12 may pass to terminal 18 signals in a lowpass frequency band, including frequency f1. FIG. 1B also shows that highpass filter 14 may pass to terminal 20 signals in a highpass frequency band, including frequency f2. The separated signals at terminals 18 and 20 may be employed by the communications system's signal processing circuitry, e.g., receiver circuitry and/or transmitter circuitry, coupled to terminals 18 and/or 20. Alternatively, the communications system's signal processing circuitry may supply signals at terminals 18 and 20 to lowpass filter 12 and to highpass filter 20, respectively, to form a combined signal at terminal 16 for transmission by the communications system.
A problem associated with the lowpass-highpass-filter architecture of diplexer 10 is that signals at undesired frequency bands may be passed by lowpass filter 12 and/or highpass filter 14. For example, referring to FIG. 1B, the desired frequency band around frequency f1 may comprise a frequency range that is less than the full range of frequencies passed by lowpass filter 12. Signals at frequencies passed by lowpass filter 12 that are outside of the desired frequency band around frequency f1 may interfere with the desired operation of the communications system. Similarly, the desired frequency band around frequency f2 may comprise a frequency range that is less than the full range of frequencies passed by highpass filter 14. Signals at frequencies passed by highpass filter 14 that are outside of the desired frequency band around frequency f2 may also interfere with desired operation of the communications system.
To address the potential interference problem, as described above for diplexer 10, some communications systems may employ a diplexer 22, as shown in FIG. 2A. Diplexer 22 includes a bandpass filter 24 and a bandpass filter 26, and may be characterized as employing a bandpass-bandpass-filter architecture. A terminal of bandpass filter 24 is coupled to a terminal 28 and the other terminal of bandpass filter 24 is coupled to a terminal 30. Similarly, a terminal of bandpass filter 26 is coupled to terminal 28 and the other terminal of bandpass filter 26 is coupled to a terminal 32. A communications system (not shown) employing diplexer 22 may include an antenna (not shown) coupled to terminal 28 and signal processing circuitry (not shown), e.g., receiver circuitry and/or transmitter circuitry, coupled to terminals 30 and/or 32.
In operation, a communications system employing diplexer 22 may receive over its antenna signals in multiple frequency bands, e.g., signals in one frequency band including frequency f1 and signals in another frequency band including frequency f2. Diplexer 22 may receive and input these signals to bandpass filter 24 and to bandpass filter 26. As represented in FIG. 2B, bandpass filter 24 may pass to terminal 30 signals in a passband frequency band, including frequency f1. FIG. 2B also shows that bandpass filter 26 may pass to terminal 32 signals in a bandpass frequency band, including frequency f2. The separated signals at terminals 30 and 32 may be employed by the communications system's signal processing circuitry, e.g., receiver circuitry and/or transmitter circuitry, coupled to terminals 30 and/or 32. Alternatively, the communications system's signal processing circuitry may supply signals at terminals 30 and 32 to bandpass filter 24 and to bandpass filter 26, respectively, to form a combined signal at terminal 28 for transmission by the communications system.
U.S. Pat. No. 6,304,156 (hereafter the “'156 patent”) and U.S. Pat. No. 6,414,567 (hereafter the “'567 patent”) disclose bandpass-bandpass-filter architectures in duplexers, similar to diplexer 22.
Regarding the '156 patent, FIG. 39 shows a circuit diagram for a duplexer 500, including a transmission filter 501 and a reception filter 502. The transmission filter 501 includes strip line resonators 511, 512, 513 that are resonators composed of front end short-circuit transmission lines shorter than the quarter wavelength. The strip line resonators 511, 512, 513 are mutually coupled by an electromagnetic field, and a band pass characteristic is provided. The reception filter 502 includes strip line resonators 521, 522, 523 that are also composed of front end short-circuit transmission lines shorter than the quarter wavelength. The strip line resonators 521, 522, 523 are also mutually coupled by an electromagnetic field, and a band pass characteristic is provided.
Regarding the '567 patent, FIG. 3 shows an electrical circuit for a duplexer 41, including resonators Q1 to Q3 that are electrically coupled to each other via coupling capacitors Cs1 and Cs2 to provide a three-stage bandpass filter BPF1. Duplexer 41 also includes resonators Q4 to Q6 that are electrically coupled to each other via coupling capacitors Cs3 and Cs4 to provide a the three-stage bandpass filter BPF2.
The bandpass-bandpass-filter architectures in the duplexers of the '156 patent and the '567 patent have at least the following disadvantages. First, when combining bandpass filters to form a duplexer, it is difficult to properly match the filter impedances to obtain the desired performance for each bandpass filter. For example, prior art duplexers may have for the channel of one bandpass filter short-circuit-like impedance at the other channel's passband. Consequently, passband performance may suffer due to the impedance mismatching often found in prior art duplexers employing bandpass-bandpass-filter architecture. Second, it is difficult to adjust a pole in a bandpass filter to provide a desired attenuation pattern, without simultaneously altering in an undesired manner another portion of the attenuation pattern. This is because adjustment of a pole in a bandpass filter tends to also move one or more other poles in the bandpass filter, which may tend to alter the attenuation pattern of the bandpass filter in an undesired manner.
Thus, there is a need for multiplexers, which overcome these and other problems of the prior art.